1. Field of Invention
This invention relates to a circuit for direct current compensation of miniature transformers.
2. Description of Prior Art
In my own U.S. Pat. No. 3,714,548, issued Jan. 30, 1973, a direct current compensation circuit for miniature transformers is described. The transformer to which that circuit is connected is typically employed to isolate telephone subscriber lines from telephone switching networks. That circuit is connected between a first winding of a transformer and a second or compensation winding of the transformer and is arranged so that any direct current flow through the first winding will result in a corresponding compensation current flow in the compensation winding.
One problem has been encountered in applying that circuit to transformers connected to an unbalanced crosspoint network. Specifically, in certain unbalanced crosspoint networks such as may be found in a PABX (private automatic branch exchange) a biasing direct current flows in the crosspoint transformer loop. The biasing direct current bias flows through the transformer winding connected to the crosspoint network which in turn produces a magnetic field in the transformer that is in the opposite magnetic direction of the magnetic field produced in the transformer by direct current flow in the windings connected in the subscriber loop. The net magnetic field in the transformer is thereby reduced and ideally, may be zero. However, although the crosspoint bias current is relatively constant, the subscriber loop current may vary. Thus, the magnetic field produced by the crosspoint winding may only partially compensate for the magnetic field produced by the subscriber loop windings.